In view of the known effect of zinc on the crystalline nature of insulin, we have investigated whether zinc alters the biologic potency of this hormone. Using the rate of lipogenesis by rat epididymal adipocytes as an index of the biologic potency of insulin, we have shown that zinc exerts a potent stimulatory effect upon lipogenesis in vitro similar to, but quite independently of, insulin and that it has an additive effect with that of insulin when both are incubated together. This effect of zinc on adipocytes and the biologic potency of insulin, hitherto unreported, is of significance at the biologic, pharmacologic, and clinical level.